Device at a radar level gauge

Abstract

A device for gauging the level of a fluid in a container with a microwave signal is described. The device comprises a vertical tube (1) mounted in the container, and a transmitter and a receiver and an electronic unit for determining the level based on the reflex position of the microwave signal. The device is characterized particularly by a calibration device in the form of a polarization means (6, 7) for polarizing the microwave signal in two polarization planes separated from each other and at least one reflecting reactance (10) in the tube (1) arranged to give a substantially stronger reflex of the microwave signal in the first polarization plane than in the second.

I claim: 
     
       1. A device for gauging the level of a fluid in a container, i.e. a fluent or granulate material, preferably a condensable gas under overpressure, comprising: vertical tube through which a number of relatively small openings in its wall so communicate with said fluid that its level in the tube is the same as in the container;   transmitter for feeding a polarizable microwave signal through the said tube, said tube having a cross section dimensioned to admit passage of the microwave signal;   an electronic unit arranged to determine the reflex position of the reflected microwave signal; characterized by     polarization means (6, 7) arranged optionally to polarize the microwave signal emitted from the transmitter in a first and a second polarization plane, which form a metal angle, preferably 90°;   at least one reflecting reactance (10) arranged in the tube (1) to give a substantially stronger reflex of the microwave signal in the fluid polarization plane than in the second.   
     
     
       2. A device according to claim 1, characterized in that the transmitter comprises a circular waveguide (5) for one-mode propagation, preferably H 11  mode. 
     
     
       3. A device according to claim 1, characterized in that the polarization means consists of a substantially plane plate (7) of a dielectric material such as a polymer, arranged with its plane substantially parallel to the symmetry axis of the waveguide, preferably crosswise filling out the diameter of the waveguide, the plate being arranged turntable about the symmetry axis of the waveguide optionally between two positions, preferably so that the planes of the plate in the two positions forms 45°. 
     
     
       4. A device according to claim 1, characterized in that the polarization means consists of at least one plane metal plate placed with its plane substantially parallel to the symmetry axis of the waveguide, not crosswise filling out the diameter of the waveguide, the metal plate being arranged turnable about the symmetry axis of the waveguide optionally between two positions, preferably so that the planes of the metal plate in the two positions form 45 °. 
     
     
       5. A device according to claim 1, characterized in that the polarization means is a section of a waveguide (3 A), which is oval in cross section, the waveguide section being arranged turntable about its symmetry axis optionally between two positions, preferably so that the longitudinal symmetry planes of the waveguide section in the two positions form 45°. 
     
     
       6. A device according to claim 1, characterized in that the said reactance is formed of a relatively thin metal pin (10), arranged with its longitudinal direction perpendicular to the longitudinal axis of the tube (1), preferably extending diametrically through the tube (1). 
     
     
       7. A device according to claim 1, characterized in that the reactance is formed by an aperture (11) in the wall of the tube, preferably with its longitudinal direction perpendicular to the longitudinal axis of the tube (1).